US10931890B2 - Image sensor system - Google Patents

Image sensor system Download PDF

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US10931890B2
US10931890B2 US16/421,250 US201916421250A US10931890B2 US 10931890 B2 US10931890 B2 US 10931890B2 US 201916421250 A US201916421250 A US 201916421250A US 10931890 B2 US10931890 B2 US 10931890B2
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binary
pixel
signal
image sensor
count value
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US20200021753A1 (en
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Laurens Korthout
Vincent Arkesteijn
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Teledyne Dalsa BV
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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N5/00—Details of television systems
    • H04N5/30—Transforming light or analogous information into electric information
    • H04N5/32—Transforming X-rays
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/60—Noise processing, e.g. detecting, correcting, reducing or removing noise
    • H04N25/617—Noise processing, e.g. detecting, correcting, reducing or removing noise for reducing electromagnetic interference, e.g. clocking noise
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
    • H01L27/00—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate
    • H01L27/14—Devices consisting of a plurality of semiconductor or other solid-state components formed in or on a common substrate including semiconductor components sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation and specially adapted either for the conversion of the energy of such radiation into electrical energy or for the control of electrical energy by such radiation
    • H01L27/142—Energy conversion devices
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/30—Circuitry of solid-state image sensors [SSIS]; Control thereof for transforming X-rays into image signals
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/40—Extracting pixel data from image sensors by controlling scanning circuits, e.g. by modifying the number of pixels sampled or to be sampled
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/70—SSIS architectures; Circuits associated therewith
    • H04N25/76—Addressed sensors, e.g. MOS or CMOS sensors
    • H04N25/77—Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components
    • H04N25/772—Pixel circuitry, e.g. memories, A/D converters, pixel amplifiers, shared circuits or shared components comprising A/D, V/T, V/F, I/T or I/F converters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04N—PICTORIAL COMMUNICATION, e.g. TELEVISION
    • H04N25/00—Circuitry of solid-state image sensors [SSIS]; Control thereof
    • H04N25/70—SSIS architectures; Circuits associated therewith
    • H04N25/76—Addressed sensors, e.g. MOS or CMOS sensors
    • H04N25/78—Readout circuits for addressed sensors, e.g. output amplifiers or A/D converters
    • H04N5/341—
    • H04N5/37455—

Definitions

  • the present invention relates to an image sensor system.
  • the invention further relates to an X-ray imaging system comprising such an image sensor system.
  • FIG. 1 An example of a system in accordance with the preamble of claim 1 is shown in FIG. 1 .
  • a matrix (only a small segment A of the matrix is shown) of photo-sensitive pixels 1 is shown in which the pixels are arranged in rows and columns.
  • This matrix comprises a plurality of column busses 2 .
  • the pixels that are arranged in the same column are connected to the same column bus 2 .
  • Signal integrating pixels are at least equipped with a photo sensitive element and a charge storage capability to collect liberated charges.
  • various transistors are included in the pixel design to control the pixel operation and support the pixel signal read-out process.
  • the output signal of the pixel is generally put onto column bus 2 when a pixel 1 is selected for read out.
  • Each row of pixels 1 may be controlled by a row controller 3 , which typically controls the pixel integration and read-out process. Generally, only a single row of pixels 1 is selected for read out at a time to prevent the situation that two pixels 1 in the same column put an output signal onto column bus 2 .
  • an appropriate signal can be read out that is representative for the amount of light energy projected on a pixel during the integration period.
  • this appropriate signal is referred to as the pixel signal.
  • correlated double sampling schemes are known to reduce the impact of pixel reset noise.
  • the pixel signal can be based on one or more different output signals for the same pixel.
  • a pixel reference level (V1) and a signal level (V2) are measured from the pixel.
  • the pixel signal is determined by subtracting these signal levels from each other in either the analog or digital domain.
  • the pixel signal may either relate to the combination of different output signals as described above or to the individual output signals.
  • the image sensor system is configured to output a series of digital numbers, wherein each digital number is representative for the amount of light energy detected by a particular pixel 1 .
  • the image sensor system comprises a conversion unit that converts the pixel signals into respective digital numbers.
  • the conversion unit may be configured to convert V1 and V2 to a respective digital number. The subtraction of the converted signal levels can then be performed in the digital domain.
  • the conversion unit comprises a global ramp unit 4 , a comparator 5 , and a binary counter 6 .
  • the global ramp unit 4 generates a ramp voltage and provides this voltage to comparator 5 .
  • comparator 5 outputs a count enable signal to binary counter 6 .
  • This counter will increase its count value provided that a falling or rising edge is detected on its clock input.
  • comparator 5 will output a count disable signal to binary counter 6 . Regardless of whether a falling or rising edge is detected at the clock input of binary counter 6 its count value will not change.
  • a comparator 5 and binary counter 6 are separately arranged for each pixel column. However, only a single global ramp unit 4 is used for distributing the ramp voltage to each comparator 5 .
  • the binary count values for the various pixel columns are collected by a binary count value collecting unit 9 , which may also be controlled by controller 7 .
  • a binary count value collecting unit 9 may be configured to output, for example in a sequential manner, the binary count values of a given row of pixels as soon as these values become available.
  • the binary count value collecting unit may comprise a memory in which the values are stored to allow the output of a single data stream that comprises data for each row of pixels.
  • controller 7 In advance of each conversion cycle, controller 7 resets binary counters 6 in each pixel column to an initial value, e.g. a zero value to allow a next conversion of pixel signals to start. These latter signals may correspond to the same row of pixels, for example because these pixels have been re-selected. However, these signals normally correspond to a different row of pixels, for example an adjacent row. To this end, controller 7 may optionally control row controller 3 to select a different row or to re-select the same row for read out, and/or to initiate a next integration period for capturing light. The controlling performed by controller 7 may be clocked using one or more clock signals. This or these clock signal(s) as well as the clock signal provided to binary counters 6 , and other components if required, is/are generated by a clock signal unit 8 that distributes one or more clock signals to the various components in the system.
  • an initial value e.g. a zero value
  • Controller 7 is generally also responsible for initiating a conversion cycle by controlling if global ramp unit 4 should start with outputting a next ramp voltage.
  • the image sensor system can be realized in CMOS technology.
  • a sensor may comprise a tile, wherein the tile corresponds to an integral part of a Si wafer.
  • the sensor may comprise a plurality of tiles that are butted.
  • the conversion unit may be arranged on each tile or the conversion unit may be located only on the bottom or top tile.
  • the conversion unit, or at least parts thereof can be realized on separate integrated circuit dies, separate from the pixel matrix, and may optionally be realized in different semiconductor technologies.
  • a known problem encountered with the image sensor system in FIG. 1 is related to the fluctuation of the current that is drawn from the supply by the binary counters during the counting process.
  • the count values for all the binary counters in the known system for which the corresponding pixel voltage is larger than the ramp voltage, i.e. the binary counter is counting are identical.
  • binary counters are used that output a binary number
  • a problem may arise when the count value changes between binary numbers such as 0111 and 1000.
  • a large number of bits will simultaneously toggle as each counting binary counter experiences the same change. Consequently, at these count values transitions, large peak currents will flow through the supply lines. If the supply current variations become too large, the behavior of other components in the system may be influenced via supply crosstalk.
  • the ground GND and supply VDD domains are shared between multiple circuits on the same chip.
  • the limited Power Supply Rejection Ratio of circuits will sense voltage drop variations on these shared supply lines introducing interferences to their outputs. These interferences can result in image quality degradation.
  • the number of toggling bits is equal to the number of gray-code counters that are still counting at that moment.
  • gray-code counters are relatively complex and require a substantially larger footprint than binary ripple counters. Consequently, gray-code counters are not the preferred solution in sensors that require a small pixel pitch.
  • the average number of bit transitions in a binary ripple counter for a counting step is roughly equal to two.
  • bit transition cancellation technique identified in the abovementioned paper cannot be applied in image sensor systems that require a small counter footprint.
  • Another known problem encountered with the image sensor system in FIG. 1 is related to the fluctuation of the current that is drawn from the supply by the binary counters when these binary counters change from operating in a counting mode to operating in a non-counting mode.
  • the binary counter increases its binary count value in dependence of the clock signal, whereas in the non-counting mode, the binary counter has stopped counting.
  • the switch between these modes depends on the outcome of the comparison process of comparing the pixel signal to the momentary ramp signal.
  • EP 2 757 776 discloses an approach wherein a set of N counters is used which set is divided into a first sub-set of counters and a second sub-set of counters.
  • a control stage is arranged to enable the first sub-set of counters or the second sub-set of counters.
  • a drawback of the approach described above is that multiple counters must be arranged in a column and different clock signals must be generated and routed adding to the complexity and costs of the system.
  • An object of the present invention is providing an image sensor system having a small counter footprint, in which the abovementioned problems associated with the variation in current drawn from the supply during counting do not occur or at least to a lesser extent.
  • the invention according to a first aspect provides an image sensor system that is characterized in that the columns of pixels are grouped in a plurality of pixel column groups, wherein each pixel column group comprises at least three columns of pixels. Furthermore, the controller is configured to reset the binary counters corresponding to the pixel columns in a pixel column group at the start of a conversion cycle such that at least three pixel columns among the at least three pixel columns in a pixel column group are reset to a different initial value.
  • the controller may be configured to simultaneously reset multiple or all binary counters.
  • the initial values to be used by each binary counter are preferably predefined in hardware at the level of each binary counter.
  • the controller is configured to trigger the binary counters to reset to the predefined initial value.
  • the initial values are stored in memory and are communicated to each binary counter by or under the supervision of the controller.
  • the binary count value collecting unit may comprise a compensating unit that is configured for subtracting the initial value corresponding to a given binary counter from the binary count value outputted by that binary counter.
  • the different initial values used in a pixel column group may correspond to a group of sequential binary numbers.
  • the m th pixel column in a pixel column group may be given as initial value the m th lowest digital number to be outputted by the binary counter. For example, when the ordering of the pixel column is from left to right, the first pixel column on the left will have the binary equivalent of the decimal number 0 as initial value, whereas the 8 th column will have the binary equivalent of the decimal number 7 as initial value.
  • the binary counter is an n-bit binary counter, such as an 11 bit or 12 bit counter, wherein the initial values are values chosen from a group consisting of the ⁇ times 2 n lowest binary numbers that can be outputted by the binary counter, wherein ⁇ is a constant preferably smaller than 0.05.
  • ⁇ is a constant preferably smaller than 0.05.
  • at least two, and more preferably each, of the pixel column groups are identical.
  • the binary count value collecting unit may be configured for sequentially obtaining the binary count values from the pixel columns, and, for each read out binary count value corresponding to a given pixel column, to subtract the initial value associated with that given pixel column from the read out binary count value.
  • the binary count value collecting unit will read out the binary count values in a particular order. This order may correspond to a left-to-right direction or vice versa. Alternatively, particular columns, corresponding to a region of interest, can be read out first. Preferably, the binary count value collecting unit will read out the binary count values on a pixel column group basis. Furthermore, the binary count value collecting unit will read out the binary count values for each pixel column in the same order as the magnitude of the initial values for that pixel column group. For example, if the matrix of pixels comprises 128 columns, divided over 8 pixel columns groups each comprising 16 pixel columns, the initial values may range from 0 to 1111 (binary equivalent of 15).
  • the binary count value collecting unit reads out the left column first and ends with the column on the right.
  • a further binary counter may be used within the binary count value collecting unit to correct for the different initial values that have been used.
  • the binary count value collecting unit may be configured to obtain a next binary count value in dependence of a clock signal.
  • the further binary counter may be configured to increase a count value outputted by the further binary counter in dependence of the clock signal.
  • the binary count value collecting unit may further comprise a subtraction unit to subtract the count value from the binary count value that is obtained next. This allows for a convenient compensation of the different initial values.
  • the clock signal to be used by or in the binary count value collecting unit may be generated by the aforementioned clock signal unit.
  • the controller may be further configured to, at the start of a conversion cycle, trigger the global ramp unit to start generating a ramp signal, such as a ramp voltage, wherein the ramp signal is preferably a single slope ramp signal.
  • a binary ripple counter is preferably used, both for the column specific binary counters and optionally for the further binary counter in the binary count value collecting unit.
  • the image sensor system may further comprise a row selecting unit for selecting a single row of pixels among the rows of pixels.
  • the row selecting unit may be controlled by the controller or may operate substantially independently based on one or more clock signals received from the clock signal unit.
  • the present invention provides an X-ray imaging system that comprises the image sensor system described above, wherein the matrix of pixels is covered by a scintillator layer for converting an incoming X-ray photon into visible light photon(s).
  • the present invention provides an image sensor system that comprises a matrix having rows and columns of photo-sensitive pixels, wherein pixels that are arranged in the same column are configured to output an output signal onto a column bus for that column if such pixel is selected for read out.
  • the system further comprises a conversion unit for converting a pixel signal that is based on one or more output signals for a given pixel into a respective digital number, wherein the conversion unit comprises a global ramp unit configured for generating a ramp signal.
  • the conversion unit further comprises, for each column of pixels, a comparator for comparing the relevant pixel signal to the ramp signal, and a binary counter configured to increase a binary count value in dependence of a clock signal and a result of said comparing.
  • the image sensor system further comprises a controller for resetting each binary counter to an initial value at the start of a conversion cycle, and a binary count value collecting unit for collecting the binary count values from each binary counter.
  • the image sensor system is characterized in that the conversion unit further comprises, for each column of pixels, a load that comprises a toggling unit configured for toggling a predefined number of bits in response to the clock signal.
  • the load and binary counter are operable in a counting mode, in which mode the binary counter increases said binary count value in dependence of the clock signal and the load does not toggle, and in a non-counting mode, in which mode the binary counter has stopped counting and the load toggles said predefined number of bits in dependence of the clock signal.
  • the predefined number corresponds to the average number of bits that toggle in the binary counter per counting step while the binary counter is in the counting mode.
  • the average number of bits that toggle in the binary counter per counting step can be computed by computing, for each possible counting step, the combined number of toggling bits in one, a plurality of the, or all binary counter(s), and to then average this combined number over the different counting steps.
  • the binary counters do not use different initial values as discussed above, the average determined for one or all of the binary counters will be identical.
  • the toggling unit may comprise a number of parallel arranged flip-flops, each flip-flop being configured to toggle a single bit. Each flip-flop may be configured to toggle, independently from the other, in dependence of the clock signal.
  • the binary counter may comprise a plurality of cascaded flip-flops, wherein the type of flip-flop used in the binary counter is identical to the type of flip-flop used in the load, the binary counter preferably being a binary ripple counter.
  • Each flip-flop in the load may be connected, at its output, to a capacitance, said capacitance corresponding to the capacitive load seen by the toggling flip-flops in the binary counter.
  • the comparator can be configured to output a count disable signal when the pixel signal is smaller than the momentary ramp signal and to output a count enable signal when the pixel signal is larger than the momentary ramp signal.
  • the binary counter may comprise an enable input, a clock input, and an output, wherein the binary counter is configured to increase its binary count value in dependence of the clock signal received at the clock input and the count enable signal being received at the enable input.
  • the load may be configured to toggle the predefined number of bits in dependence of the clock signal and whether the load receives the count disable signal.
  • the columns of pixels may be grouped in a plurality of pixel column groups, each pixel column group comprising at least three columns of pixels, wherein the controller is configured to reset the binary counters corresponding to the pixel columns in a pixel column group at the start of a conversion cycle such that at least three pixel columns among the at least three pixel columns in a pixel column group are reset to a different initial value.
  • At least two, and preferably each, of the column groups may be identical.
  • the number of predefined bits may correspond to the average number of toggling bits per counting step per binary counter in the pixel column group.
  • the average number of toggling bits per counting step computed per binary counter in the pixel column group may deviate from the average computed per single binary counter. This is due to the different initial values that are used in the pixel column group. For example, a first binary counter may toggle 3 bits for a given counting step whereas a second binary counter in the same pixel column group may toggle 1 bit due to the different initial value. Consequently, the average number of toggling bits for that counting step may be reduced compared to cases wherein all binary counters have identical initial values.
  • the initial values and the number of pixel columns in a pixel column group may be such that a variation over time in the number of bits toggling per counting step in a pixel column group is reduced compared to a variation over time in the number of bits toggling per counting step for a single pixel column.
  • the initial values to be used by each binary counter can be predefined in hardware at the level of each binary counter, in which case the controller can be configured to trigger the binary counters to reset to the predefined initial value.
  • the binary count value collecting unit may comprise a compensating unit that is configured for subtracting the initial value corresponding to a given binary counter from the binary count value outputted by that binary counter.
  • the different initial values may correspond to a group of sequentially arranged binary numbers.
  • the controller can be configured to set, as initial value for an m th pixel column in a pixel column group, the m th lowest digital number to be outputted by the binary counter associated with that pixel column.
  • the controller can be configured to simultaneously reset multiple or all binary counters.
  • the binary counter is an n-bit binary counter, wherein the initial values are values chosen from a group consisting of the ⁇ times 2 n lowest binary numbers that can be outputted by the binary counter, wherein a is a constant preferably smaller than 0.05.
  • the binary count value collecting unit can be configured for sequentially obtaining the binary count values from the pixel columns, and, for each read out binary count value corresponding to a given pixel column, to subtract the initial value associated with that given pixel column from the read out binary count value.
  • the binary count value collecting unit is preferably configured to obtain a next binary count value in dependence of a clock signal, wherein the binary count value collecting unit further comprises a further binary counter that is configured to increase a count value outputted by the further binary counter in dependence of said clock signal, and a subtraction unit to subtract the count value from the binary count value that is obtained next.
  • the controller can be further configured to, at the start of a conversion cycle, trigger the global ramp unit to start generating the ramp signal, the ramp signal preferably being a single slope ramp signal.
  • the image sensor system further comprises a row selecting unit for selecting a single row of pixels among the rows of pixels.
  • the present invention provides an X-ray imaging system that comprises the image sensor system described above in conjunction with the third aspect of the invention, wherein the matrix of pixels is covered by a scintillator layer for converting an incoming X-ray photon into visible light photon(s).
  • FIG. 1 illustrates a known image sensor system
  • FIG. 2 illustrates an embodiment of the invention showing the binary count value collecting unit
  • FIGS. 3A and 3B illustrate the variation in the average number of toggling bits as a function of the digital number for a pixel column group consisting of a single pixel column and 32 pixel columns, respectively;
  • FIG. 4 illustrates an embodiment that addresses fluctuation in current drawn from the supply due to the transition between the counting mode and the non-counting mode.
  • FIG. 2 illustrates an embodiment of the invention in which binary count value collecting unit 9 comprises a collector 91 for collecting the binary count values from binary counters 6 , a further binary counter 93 , a subtracting unit 94 , and a controller 92 .
  • further binary counter 93 is configured to count from 0 (binary) to 1111 (binary).
  • the 1024 pixel columns are grouped in 64 pixel column groups, each group comprising 16 pixel columns.
  • the pixel columns are numbered left to right, starting with 0 and ending with 15.
  • Each pixel column is associated with an initial value to be used by binary counter 6 in that group that equals the binary equivalent of the pixel column number. Accordingly, the 25 th pixel column corresponds to the second pixel column group, and in that pixel column group, it will be given pixel column number 9 and will consequently be associated with initial value 1000.
  • the pixel signal corresponds to a pixel voltage.
  • the invention is not limited to voltages and other electrical signals, such as currents or electrical charges, or combinations thereof may be used.
  • Controller 7 will reset binary counters 6 to the corresponding initial values as described above and will instruct or trigger global ramp unit 4 to generate a ramp voltage that is distributed to comparator 5 in each pixel column.
  • This trigger or instruction will preferably by synchronized to or with the clock signal generated by clock signal unit 8 .
  • comparator 5 will output a count enable signal, e.g.
  • a high logical value to binary counter 6 if the momentary ramp voltage is smaller than or equal to the pixel voltage, and will output a count disable signal, e.g. a low logical value, if the momentary ramp voltage is larger than the pixel voltage.
  • the ramp voltage and the binary counters are matched and synchronized in the sense that by the time the binary counter has received a number of rising edges that corresponds to the maximum number to be outputted by that counter, the ramp voltage has reached its end voltage.
  • the present invention is not limited to linear ramp signals. Curved ramp signals may equally be used to attribute more weight to either low or high pixel signals.
  • each binary counter may be equipped with a memory to store the final binary count value while still being able to perform a counting operation in connection with a next conversion cycle.
  • buffering may be implemented in binary count value collecting unit 9 .
  • further controller 92 instructs further binary counter 93 to start counting from 0 to 1111 (binary). At the same time, it will instruct collector 91 to output the count value for a particular pixel column. Preferably, the process will start by reading out the outer left pixel column. Typically this process is synchronized using a clock signal that may optionally be generated by clock signal unit 8 . Alternatively, this signal is generated by a dedicated clock signal inside binary count value collecting unit 9 .
  • the output of further binary counter 93 and the count value obtained from collector 91 are fed to subtracting unit 94 .
  • the count value of further binary counter 93 is subtracted from the obtained count value.
  • the count value of further binary counter 93 is increased. In this manner, the initial value that was used for a given pixel column can be compensated.
  • Subtracting unit 94 may be configured to output a stream of corrected count values as indicated by arrow 95 .
  • the corrected count values are stored in a memory in binary count value correcting unit 9 .
  • the count values for all the pixels of the matrix may be stored in a memory in binary count value correcting unit 9 prior to outputting these values as a stream.
  • FIGS. 3A and 3B illustrate the variation in the average number of toggling bits, per pixel column, as a function of the binary count for a pixel column group consisting of a single pixel column and 32 pixel columns, respectively.
  • a pixel column group consists of 32 pixel columns
  • the variation in the number of toggling bits over the various count values is considerably less than when the pixel column group only consists of a single pixel column.
  • increasing the number of the pixel columns comes at a cost as the number of digital numbers that is available for signal quantization is reduced by the amount of different initial values.
  • a significant reduction can already be obtained with relatively small pixel column groups, a strong reduction in the supply current variation can be achieved without substantially degrading the effective dynamical range of the binary counter.
  • a binary counter can, during a conversion process of pixel signals for a given row of pixels, be in a counting mode in which mode the binary counter increases its binary count value in dependence of the clock signal.
  • the binary counter can also be in a non-counting mode, in which mode the binary counter has stopped counting.
  • the mode in which the binary counter operates depends on the comparison between the ramp signal and the relevant pixel signal. For example, when the pixel signal in the form of a pixel voltage is smaller than the momentary ramp signal in the form of a ramp voltage, a count disable signal is outputted by the comparator. Similarly, when the pixel voltage is larger than the momentary ramp voltage, a count enable signal is outputted by the comparator.
  • the count enable signal or count disable signal is fed to the binary counter, which will increase its binary count value when a positive edge is detected in the clock signal and the count enable signal is received.
  • Using pixel column groups addresses the variation in the number of toggling bits when the binary counters in that group are in the counting mode. However, whenever a binary counter switches between the counting mode to the non-counting mode a similar change in current drawn from the supply occurs. To address this problem, the present invention proposes a solution in which a dummy load is used to reduce the variation in current drawn from the supply caused by the transitions between the counting mode and the non-counting mode. This solution will be described next under reference to FIG. 4 .
  • FIG. 4 illustrates an embodiment of a binary ripple counter 6 .
  • This counter comprises a plurality 61 of cascaded flip-flops and an initial value unit 62 that holds the initial value to be used by this counter.
  • Binary counter 6 has three inputs, i.e. a set input, a disable/enable signal input, and a clock input. Many different ways to implement binary counter 6 are known in the art.
  • controller 7 If an instruction to set binary counter 6 is provided by controller 7 using a set signal (SET) provided to the set input of binary counter 6 , the output of the plurality 61 of flip-flops will be set equal to the initial value using initial value unit 62 .
  • SET set signal
  • binary counter 61 is configured to increase its binary count value if a rising edge is detected in the clock signal (CLK) provided at its clock input and if the disable/enable signal (DISA/ENA) received at the corresponding input indicates that the pixel signal is larger than the momentary ramp signal.
  • CLK clock signal
  • DISA/ENA disable/enable signal
  • a load 63 is connected in parallel to counter 6 .
  • This load comprises a number of flip-flops 64 , preferably the same type of flip-flops as used in the plurality 61 of flip-flops in counter 6 .
  • Load 63 is configured to toggle a predefined number of bits in response to the clock signal (CLK), provided that the output of comparator 5 is indicative for the situation wherein the momentary ramp signal is larger than the pixel signal and counter 6 is no longer counting. More in particular, load 63 is configured to perform said toggling when a triggering edge, typically the rising edge, is detected in the received clock signal (CLK).
  • the number of toggling bits, two in FIG. 4 should be chosen such that this number corresponds to the average number of toggling bits per counting step of counter 6 .
  • this number corresponds to the average number of toggling bits per counting step of counter 6 .
  • two flip-flops 64 are used in the embodiment in FIG. 4 . Still, in other embodiments wherein fewer pixel columns are present in a pixel column group, or even wherein no pixel column groups are used at all, a different number of flip-flops may be used.
  • load 63 comprises a toggling unit configured for toggling a predefined number of bits in response to the clock signal, wherein the predefined number corresponds to the average number of bits that toggle in the binary counter per counting step while being in the counting mode.
  • flip-flops 64 may each be connected, at their non-inverting outputs, to respective capacitances C.
  • capacitances may be connected to the input of flip-flops 64 .
  • the combination of the flip-flops 64 and the capacitances at the output, and optionally also at the input, should resemble the electrical switching behavior of the toggling flip-flops in the plurality 61 of flip-flops as much as possible to cause the same current to be drawn from the supply. Ideally, from the perspective of the supply, it should not or hardly be noticeable whether counter 6 is counting or not merely by looking at the overall current drawn from the supply.
  • load 63 has been described in conjunction with the use of pixel column groups, it should be noted that load 63 can be applied independently therefrom. More in particular, the use of pixel column groups reduces the variation over time in the number of toggling bits, and therefore of the variation over time of the current drawn from the supply, when the binary counters are counting. The use of load 63 reduces the variation in the current drawn from the supply due to binary counters making the transition from the counting mode to the non-counting mode. Although preferably used simultaneously, the pixel column groups and loads can be used independently from each other.

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US20080135729A1 (en) * 2006-10-13 2008-06-12 Sony Corporation Solid-image-pickup device, image-pickup device, and method of driving solid-image-pickup device
US20140266991A1 (en) * 2013-03-14 2014-09-18 Taiwan Semiconductor Manufacturing Co., Ltd. Systems and Methods to Mitigate Transient Current for Sensors
WO2017076748A1 (fr) 2015-11-06 2017-05-11 Cmosis Bvba Conversion analogique-numérique et procédé de conversion analogique-numérique

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GB201300999D0 (en) 2013-01-21 2013-03-06 Cmosis Nv Analog-to-digital conversation in pixel arrays

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Publication number Priority date Publication date Assignee Title
US20080135729A1 (en) * 2006-10-13 2008-06-12 Sony Corporation Solid-image-pickup device, image-pickup device, and method of driving solid-image-pickup device
US20140266991A1 (en) * 2013-03-14 2014-09-18 Taiwan Semiconductor Manufacturing Co., Ltd. Systems and Methods to Mitigate Transient Current for Sensors
WO2017076748A1 (fr) 2015-11-06 2017-05-11 Cmosis Bvba Conversion analogique-numérique et procédé de conversion analogique-numérique

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